In many parts of the world abundant supplies of natural gas are found in areas where there is little or no demand for the natural gas. This natural gas is a valuable product if available at many market places in the world. Accordingly, various ways of transporting this natural gas to areas having a market place have been considered. In some instances pipelines have been used but in other instances the distances are prohibitive for use of pipelines.
In such instances, the natural gas has been either converted to a liquid product by processes such as the Fischer-Tropsch process or made into a liquid by natural gas liquefaction processes. Such processes are well known and constitute mixed refrigerant processes such as shown in U.S. Pat. No. 4,033,735 (the '735 patent) issued Jul. 5, 1977 to Leonard K. Swenson and assigned to J. F. Pritchard and Company. A further process of this type is shown in U.S. Pat. No. 5,657,643 (the '643 patent) issued Aug. 19, 1997 to Brian C. Price and assigned to The Pritchard Corporation.
Other processes, such as cascade refrigerant processes and the like are also known for such liquefaction processes. Such processes are designed, however, for the liquefaction of large volumes of natural gas and typically comprise systems wherein a refrigerant or a plurality of refrigerants are compressed by centrifugal compressors, typically turbine compressors, which may be driven by light hydrocarbon gas fueled turbines or electric motors and thereafter passed to cooling and separation. The separated refrigerant components may then be recombined and passed to a refrigeration section. The refrigerant components or other refrigerants may be passed to the refrigeration section with or without mixing and the like.
A wide variety of processes have been used to liquefy natural gas. In the processes discussed in the patents referred to above, a mixed refrigerant is passed through a heat exchange path and cooled in a refrigeration zone, flashed to further cool the stream which is then allowed to evaporate progressively as it moves through the refrigeration section to produce a vaporous refrigerant with the refrigeration section being cooled by the heat of evaporation of the mixed refrigerant as it moves through the refrigeration section. This passage of the vaporizing refrigerant serves to cool the mixed refrigerant as it passes through the refrigeration section with the natural gas being liquefied by passing it through the refrigeration section, optionally removing it from the refrigeration section at some intermediate point along its cooling path to remove heavier materials, such as C3 and heavier hydrocarbons and the like, from the natural gas and then passing the natural gas back into the refrigeration section for further cooling to produce an at least a liquefied LNG stream. This liquefied LNG stream may be cooled further by flashing gas from the body of the liquefied natural gas if desired to further reduce the temperature of the LNG. This flash gas is typically boil-off gas and is approximately of the same composition as the rest of the LNG. Clearly the amount of boil-off gas which is flashed in this fashion is quite variable depending upon the amount of LNG produced. The boil-off gas so produced may be re-liquefied, passed to fuel and the like. It is very suitable for use as a stream for return to the refrigeration section but such requires compression and other steps which may not be available at unloading facilities and the like in all instances. Boil-off gas is lost during operations such as the unloading of tanker vessels, the storage of natural gas in insulated tanks or underground storage areas and the like where re-liquefaction facilities may not be available.
The amount of boil-off gas which is lost can be widely variable and as a result it has been difficult to suitably recover and re-liquefy boil-off gas in areas in which liquefaction facilities do not exist, such as in unloading areas for LNG tankers, for the passage of LNG into storage areas such as tanks and underground storage and the like. In many instances boil-off gas is also released as the LNG is removed from storage for re-gasification to produce gas to feed a pipeline.
This valuable boil-off gas resource is suitable for re-liquefaction but is not normally used for such purposes except possibly in areas where liquefaction facilities exist. Accordingly, since this boil-off gas represents a valuable resource a continuing effort has been directed to methods for the recovery of the boil-off gas.